Secondary battery and a method for fabricating the same

ABSTRACT

The adhesion between metal foil serving as a current collector and a negative electrode active material is increased to enable long-term reliability. An electrode active material layer (including a negative electrode active material or a positive electrode active material) is formed over a base, a metal film is formed over the electrode active material layer by sputtering, and then the base and the electrode active material layer are separated at the interface therebetween; thus, an electrode is formed. The electrode active material particles in contact with the metal film are bonded by being covered with the metal film formed by the sputtering. The electrode active material is used for at least one of a pair of electrodes (a negative electrode or a positive electrode) in a lithium-ion secondary battery.

TECHNICAL FIELD

The present invention relates to a structure of a secondary battery anda method for fabricating the secondary battery. In particular, thepresent invention relates to an electrode of a lithium-ion secondarybattery.

BACKGROUND ART

Examples of the secondary battery include a nickel-metal hydridebattery, a lead-acid battery, and a lithium-ion secondary battery.

Such secondary batteries are used as power sources in portableinformation terminals typified by mobile phones. In particular,lithium-ion secondary batteries have been actively researched anddeveloped because capacity thereof can be increased and size thereof canbe reduced.

Patent Document 1 discloses that a multilayer graphene flake ormultilayer graphene flakes are wrapped around positive electrode activematerial particles or negative electrode active material particles toprevent dispersion of the positive electrode active material particlesor the negative electrode active material particles and collapse of apositive electrode active material layer or a negative electrode activematerial layer. Multilayer graphene can maintain the bond to thepositive electrode active material particles or the negative electrodeactive material particles.

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2013-028526 DISCLOSURE OF INVENTION

In a lithium-ion secondary battery, a carbon material is typically usedas a negative electrode active material, and lithium metal complex oxideis typically used as a positive electrode active material.

When an electrode layer is formed over a current collector such as metalfoil, slurry in which fine particles of a positive electrode activematerial or a negative electrode active material are suspended in asolvent (suspension containing a binder and the like) is applied ontothe metal foil and dried. For example, in the case of forming a negativeelectrode, a solution containing carbon particles is applied onto copperfoil or aluminum foil and dried; moreover, this is pressed if necessary.The negative electrode formed in such a manner, a separator, a positiveelectrode containing a positive electrode active material such aslithium iron phosphate, and an electrolytic solution are assembled tofabricate a lithium-ion secondary battery.

The type of deterioration of the lithium-ion secondary battery can bebroadly classified as and expressed by the terms “calendar life” and“cycle life”. The term “calendar life” is used to express deteriorationdue to an electrochemical change caused by charging a lithium-ionsecondary battery at high temperature even after the lithium-ionsecondary battery is fully charged. The term “cycle life” is used toexpress deterioration due to an electrochemical change or a physicalchange of a lithium-ion secondary battery caused by repeating charge anddischarge.

There are some factors that influence the deterioration expressed by theterms “cycle life” and “calendar life”.

For example, a factor that influences the deterioration is a binder.Typically, an organic material such as polyvinylidene fluoride is usedas the binder. The adhesion of metal foil such as copper foil oraluminum foil (a base) to polyvinylidene fluoride (the binder) is notsufficient at the interface between the metal foil and the binder. Whenused by itself, the binder is a cause of the internal resistance of abattery. Therefore, the used amount of the binder is preferably small.

Another factor that influences the deterioration is carbon particles.Surfaces of the carbon particles are extremely water repellent. An areaof contact between metal foil and the carbon particles is small, whichindicates that the metal foil and the carbon particles are in pointcontact with each other, therefore, it is difficult to ensure sufficientadhesion. Furthermore, it is known that the carbon particle has anapproximately 10% volume change by intercalation and deintercalation oflithium, and stress is generated at the interface between a currentcollector and the carbon particles that are active material particles.For these reasons, when a lithium-ion secondary battery is charged anddischarged repeatedly, the adhesion of the metal foil to the negativeelectrode active material is decreased, so that the negative electrodeactive material is separated from the metal foil, decreasing charge anddischarge characteristics or shortening the life of the lithium-ionsecondary battery.

One object is to increase the adhesion between metal foil serving as acurrent collector and a negative electrode active material in alithium-ion secondary battery to ensure long-term reliability.

Another object is to provide a novel structure of an electrode in alithium-ion secondary battery. Another object is to provide a flexiblesecondary battery.

An electrode active material layer (including a negative electrodeactive material or a positive electrode active material) is formed overa base, a metal film is formed over the electrode active material layerby sputtering among other methods, and then the base and the electrodeactive material layer are separated at the interface therebetween; thus,an electrode is formed. The electrode active material particles incontact with the metal film are bonded by being covered with the metalfilm formed, for example, by the sputtering; the electrode activematerial layer is used for at least one of a pair of electrodes (anegative electrode or a positive electrode) in the lithium-ion secondarybattery.

One embodiment of the invention disclosed in this specification is amethod for fabricating a secondary battery including the steps ofapplying slurry containing electrode active material particles onto abase and drying the slurry, forming a metal film by sputtering amongother methods to bond the electrode active material particles with eachother or strengthen the bond between the electrode active materialparticles, and separating the base and the electrode active materialparticles from each other at the interface therebetween, thereby formingan electrode including the electrode active material particles that arepartly bonded with each other with the metal film.

The lithium-ion secondary battery fabricated in such a manner has anovel structure. The structure of the secondary battery includes a firstelectrode that includes a plurality of electrode active materialparticles and a metal film that bonds the electrode active materialparticles adjacent to each other, a second electrode, and anelectrolytic solution at least between the first electrode and thesecond electrode.

In addition, the metal film is not limited to a metal film that bondsadjacent particles in the two-dimensional direction. The metal film mayextend into the space between the adjacent particles, and bond theadjacent particles in the film thickness direction. In this structure, asecondary battery includes a pair of electrodes and an electrolyticsolution provided therebetween. At least one of the pair of electrodesincludes electrode active material particles and a metal film that fillsat least part of a space between the electrode active materialparticles. The metal film that partly fills the space bonds theelectrode active material particles with each other or strengthens thebond between the electrode active material particles.

In the case of using carbon particles as a negative electrode activematerial, for example, the carbon particles are provided on a surface ofa flat base, and a space is provided between the carbon particles. Ametal film is formed by sputtering to fill at least part of the spacebetween the carbon particles. When a layer containing carbon particlesis referred to as a negative electrode active material layer, aplurality of projections and a plurality of depressions are provided ona surface of the negative electrode active material layer. In this case,a metal film is formed by sputtering to fill the plurality ofdepressions on the surface of the negative electrode active materiallayer or to planarize the plurality of projections on the surface of thenegative electrode active material layer.

The metal film formed by sputtering maintains the bond between thenegative electrode active material particles that are in contact withthe metal film. When copper is used as the material of the metal film,the metal film serves as a current collector.

Also in the case where a positive electrode has active materialparticles, a metal film is formed by sputtering to fill at least part ofa space between positive electrode active material particles. The formedmetal film maintains the bond between the positive electrode activematerial particles that are in contact with the metal film. When copperis used as the material of the metal film, the metal film serves as acurrent collector.

Furthermore, if necessary, a current collector is fixed to andelectrically connected to the metal film, whereby the current collectorand the metal film can be in surface contact with each other, resultingin an increase in the adhesion therebetween. In the case of providingthe current collector in contact with the metal film, the metal filmserves as a buffer layer between the current collector and the negativeelectrode active material (or the positive electrode active material).

A method for fabricating a secondary battery in the case of fixing acurrent collector to a metal film is as follows. Slurry containingelectrode active material particles is applied onto a base and dried, ametal film is formed by sputtering to bond the electrode active materialparticles with each other or strengthen the bond between the electrodeactive material particles, the base and the electrode active materialparticles are separated from each other at the interface therebetween,and the current collector is electrically connected to the metal film,thereby forming an electrode. By strengthening the bond between theelectrode active material particles, the adhesion between the currentcollector and the electrode active material particles can be kept evenwhen the current collector and the electrode active material are bent.Consequently, a flexible secondary battery can be provided.

The flexible secondary battery can have a curved surface with acurvature radius of greater than or equal to 10 mm and less than orequal to 150 mm.

Description is given of the radius of curvature of a surface withreference to FIGS. 11A to 11C. In FIG. 11A, in a plane surface 1701along which a curved surface 1700 is cut, part of a curve 1702 isapproximate to an arc of a circle, and the radius of the circle isdenoted by a radius 1703 of curvature and the center of the circle isdenoted by a center 1704 of curvature. FIG. 11B is a top view of thecurved surface 1700. FIG. 11C is a cross-sectional view of the curvedsurface 1700 taken along the plane surface 1701. When a curved surfaceis cut along a plane surface, the radius of curvature of a curve dependson along which plane surface the curved surface is cut. Here, the radiusof curvature of the surface is defined as the radius of curvature of acurve which is cut along a plane surface to have a curve with thesmallest radius of curvature.

In the case of curving a secondary battery in which a component 1805including electrodes and an electrolytic solution is sandwiched betweentwo films as exterior bodies, a radius 1802 of curvature of a film 1801close to a center 1800 of curvature of the secondary battery is smallerthan a radius 1804 of curvature of a film 1803 far from the center 1800of curvature (FIG. 10A). When the secondary battery is curved and has anarc-shaped cross section, compressive stress is applied to a surface ofthe film close to the center 1800 of curvature and tensile stress isapplied to a surface of the film far from the center 1800 of curvature(FIG. 10B). However, by forming a pattern of projections and depressionson surfaces of the exterior bodies, influence of distortion can bereduced to be acceptable even when the compressive stress and thetensile stress are applied. For this reason, the secondary battery canbe deformed as long as the exterior body close to the center ofcurvature has a curvature radius of greater than or equal to 10 mm,preferably greater than or equal to 30 mm.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partiallyarc-shaped; for example, a shape illustrated in FIG. 10C, a wavy shapeillustrated in FIG. 10D, and an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can be deformed as long as a surfaceof the exterior body close to the center of curvature has a curvatureradius of greater than or equal to 10 mm, preferably greater than orequal to 30 mm in a curved surface having the smallest radius ofcurvature among the radiuses of curvature of the centers.

As a material of the base, a material which can withstand sputtering forforming the metal film can be used; in addition, the material hardlyreacts with a solvent containing the negative electrode active material.For example, a plastic film or metal foil (e.g., titanium foil andcopper foil) can be used. Furthermore, to separate the electrode activematerial particles from the base easily in the later step, a siliconoxide film or a fluororesin film (e.g., a polytetrafluoroethylene film)may be provided on a surface of the plastic film or a surface of themetal foil. The metal film can be formed by a known method, e.g.,sputtering, evaporation, and chemical vapor deposition.

As the negative electrode active material, carbon particles aretypically used; for example, natural graphite (e.g., scale-like andspherical) and artificial graphite can be used. Note that carbonparticles whose surfaces are partly covered with a silicon oxide filmmay be used as the negative electrode active material, for example.

In this specification, there is no particular limitation on the negativeelectrode active material as long as lithium ions can electrochemicallyoccluded into and released from the negative electrode active material.

Note that the metal film is not in contact with all the negativeelectrode active material particles except for the case where the numberof the negative electrode active material particles is small. Therefore,a binder may be used to strengthen the bond between the active materialparticles, or a conductive additive may be used to increase electricconductivity between the active material particles or between the activematerial and the current collector.

The bond between the negative electrode active material particles can bepartly strengthened with the use of the metal film; consequently, theused amount of a binder can be smaller than that in the case where themetal film is not used.

In addition, to strengthen the bond between the active materialparticles, a plurality of graphene flakes may be formed in such a way asto wrap or coat a plurality of active material particles. Graphene is acarbon material having a crystal structure in which hexagonal skeletonsof carbon are spread in a planar form and is one atomic plane extractedfrom graphite crystals. Due to its electrical, mechanical, or chemicalcharacteristics which are surprisingly excellent, the graphene has beenexpected to be used for a variety of fields of, for example,field-effect transistors with high mobility, highly sensitive sensors,highly-efficient solar cells, and next-generation transparent conductivefilms and has attracted a great deal of attention. When the plurality ofgraphene flakes are formed, the metal film formed by sputteringstrengthens the bond between graphene and the active material.

In this specification, the term “slurry” refers to suspension in whichan electrode active material is suspended in a solvent, and suspensionwhich contains not just the solvent but other additives such as abinder, a conductive additive, and graphene oxide is also referred to as“slurry”.

In the case of using active material particles, a metal film formed bysputtering can inhibit the active material particles and a currentcollector from being separated, which makes it possible to provide alithium-ion secondary battery with sufficient charge and dischargecharacteristics and long-term reliability. Furthermore, a flexiblesecondary battery including a flexible electrode can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1E are cross-sectional views illustrating steps of oneembodiment of the present invention.

FIG. 2 is a cross-sectional SEM image of one embodiment of the presentinvention.

FIGS. 3A to 3D are cross-sectional views illustrating steps of oneembodiment of the present invention.

FIGS. 4A to 4C are cross-sectional views illustrating steps of oneembodiment of the present invention.

FIGS. 5A to 5C illustrate a coin-type storage battery.

FIG. 6 illustrates a laminated storage battery.

FIGS. 7A and 7B illustrate a cylindrical storage battery.

FIGS. 8A and 8B illustrate electronic devices.

FIGS. 9A and 9B illustrate an electronic device.

FIGS. 10A to 10D illustrate a center of curvature.

FIGS. 11A to 11C illustrate a radius of curvature of a surface.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. In addition, the present invention should notbe construed as being limited to the description in the followingembodiments.

Embodiment 1

Description is given below of a method for forming an electrode of alithium-ion secondary battery of one embodiment of the present inventionwith reference to FIGS. 1A to 1E.

First, slurry containing an electrode active material 102 is appliedonto a base 100 and dried. FIG. 1A is a cross-sectional schematic viewof a state in which the slurry containing the electrode active material102 is applied onto the base 100 and dried.

In this embodiment, steps of forming a negative electrode with the useof a carbon-based material as the electrode active material 102 aredescribed below. Note that in FIG. 1A, the electrode active material 102is electrode active material particles made of secondary particleshaving an average particle diameter and a particle size distribution.For this reason, the electrode active material 102 is schematicallyillustrated as spheres in FIG. 1A; however, the shape of the electrodeactive material 102 is not limited to this shape.

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, and carbon black. Examples of the graphite includeartificial graphite such as meso-carbon microbeads (MCMB), coke-basedartificial graphite, or pitch-based artificial graphite and naturalgraphite such as spherical natural graphite. In addition, the shape ofthe graphite is a flaky shape or a spherical shape, for example.

In this embodiment, copper foil is used as the base 100, and a mixtureof MCMB and an ethyl silicate solution is used as the slurry.

A material that hardly reacts with a solvent contained in the slurry andhas low adhesion to the electrode active material 102 is used for thebase 100. Furthermore, the material of the base 100 can be deposited bysputtering in a vacuum in the later step. As the base 100, a polyimidefilm, a glass substrate, and copper foil can be used. A fluororesin filmor a silicon oxide film may be formed on the surface of the polyimidefilm, the glass substrate, or the copper foil.

If necessary, pressing may be performed after the drying.

Next, as illustrated in FIG. 1B, a metal film 101 is formed over theelectrode active material 102. Sputtering is used for the filmformation. In this embodiment, a titanium film with a thickness of 1 μmor greater, here a 3-μm-thick titanium film, is formed as the metal film101. In this embodiment, a substrate temperature is a room temperature,pressure is 0.3 Pa, and the flow rate of an argon gas is 7.5 sccm.

FIG. 2 is a cross-sectional scanning electron microscope (SEM) image ofFIG. 1B. In FIG. 2, an interface between the metal film 101 and theelectrode active material 102 is observed and part of a surface ofgraphite is found to be in contact with a titanium film.

Next, as illustrated in FIG. 1C, the base 100 and the electrode activematerial 102 are separated at the interface therebetween. The lowadhesion between the base 100 and the electrode active material 102 ispreferable to the separation; however, there is no problem in separatingthe electrode active material 102 from the base 100 with part of theelectrode active material 102 remaining on a surface of the base 100.

FIG. 1D illustrates the state after the separation. When the structurein FIG. 1D has enough mechanical strength, the structure can be used asthe negative electrode. In this case, a film with high conductivity isused as the metal film 101 because the metal film 101 serves as acurrent collector.

Next, as illustrated in FIG. 1E, the metal film 101 is electricallyconnected to a current collector 104.

Note that the current collector 104 can be formed using a highlyconductive material which is not alloyed with a carrier ion such as alithium ion, e.g., a metal typified by stainless steel, gold, platinum,zinc, iron, copper, aluminum, titanium, tantalum and an alloy thereof.Alternatively, an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, andmolybdenum, is added can be used. Still alternatively, a metal elementwhich forms silicide by reacting with silicon can be used. Examples ofthe metal element which forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, and nickel. The current collector 104 canhave, for example, a foil-like shape, a plate-like shape (sheet-likeshape), a net-like shape, a cylindrical shape, a coil shape, apunching-metal shape, and an expanded-metal shape, as appropriate. Thecurrent collector 104 preferably has a thickness of greater than orequal to 10 μm and less than or equal to 30 μm.

Through the above steps, the negative electrode of the lithium-ionsecondary battery can be formed.

To increase the adhesion between the current collector 104 and theelectrode active material 102, the metal film 101 formed by sputteringis used as a buffer layer; thus, the lithium-ion secondary battery canhave high reliability.

Embodiment 2

In this embodiment, steps of forming a positive electrode with the useof LiFePO₄ having an olivine crystal structure as an electrode activematerial particle 202 are described below. LiFePO₄ is particularlypreferable because it properly has properties necessary for the positiveelectrode active material, such as safety, stability, high capacitydensity, high potential, and the existence of lithium ions which can beextracted in initial oxidation (charging).

For the electrode active material particle 202, a material into and fromwhich lithium ions can be inserted and extracted can be used. Forexample, a lithium-containing material with an olivine crystalstructure, a layered rock-salt crystal structure, and a spinel crystalstructure can be used. As the positive electrode active material, acompound such as LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, and MnO₂can be used.

Typical examples of the lithium-containing material with an olivinecrystal structure represented by a general formula, LiMPO₄ (M is one ormore of Fe(II), Mn(II), Co(II), and Ni(II)), are LiFePO₄, LiNiPO₄,LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1,and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Examples of the lithium-containing material with a layered rock-saltcrystal structure include lithium cobalt oxide (LiCoO₂); LiNiO₂; LiMnO₂;Li₂MnO₃; an NiCo-based lithium-containing material (a general formulathereof is LiNi_(x)Co_(1-x)O₂ (0<x<1)) such as or LiNi_(0.8)Co_(0.2)O₂;an NiMn-based lithium-containing material (a general formula thereof isLiNi_(x)Mn_(1-x)O₂ (0<x<1)) such as LiNi_(0.5)Mn_(0.5)O₂; and anNiMnCo-based lithium-containing material (also referred to as NMC, and ageneral formula thereof is LiNi_(x)Mn_(y)Co_(1-x-y)O₂ (x>0, y>0, x+y<1))such as LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂. Moreover, the examples furtherinclude Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂ and Li₂MnO₃-LiMO₂ (M=Co, Ni, orMn).

Examples of the lithium-containing material with a spinel crystalstructure include LiMn₂O₄, Li_(1-x)Mn_(2-x)O₄, LiMn_((2-x))Al_(x)O₄, andLiMn_(1.5)Ni_(0.5)O₄.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1-x)MO₂ (M=Co, Al, for example)) to a lithium-containingmaterial with a spinel crystal structure which contains manganese suchas LiMn₂O₄ because advantages such as minimization of the elution ofmanganese and the decomposition of an electrolytic solution can beobtained.

Alternatively, a lithium-containing material represented by a generalformula, Li_((2-j))MSiO₄ (M is one or more of Fe(II), Mn(II), Co(II),and Ni(II), 0≦j≦2), can be used as the positive electrode activematerial. Typical examples of the general formula, Li_((2-j))MSiO₄,include Li_((2-j))FeSiO₄, Li_((2-j))NiSiO₄, Li_((2-j))CoSiO₄,Li_((2-j))MnSiO₄, Li_((2-j))Fe_(k)Ni_(l)SiO₄,Li_((2-j))Fe_(k)Co_(l)SiO₄, Li_((2-j))Fe_(k)Mn_(l)SiO₄,Li_((2-j))Ni_(k)Co_(l)SiO₄, Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+≦l≦1, 0<k<1,and 0<l<1), Li_((2-j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2-j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≦1, 0<m<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a NASICON compound represented by a generalformula, A_(x)M₂(XO₄)₃ (A=Li, Na, or Mg, M=Fe, Mn, T, V, Nb, or Al, andX=S, P, Mo, W, As, or Si), can be used as the positive electrode activematerial. Examples of the NASICON compound include Fe₂(MnO₄)₃,Fe₂(SO₄)₃, and Li₃Fe₂(PO₄)₃. Still further alternatively, compoundsrepresented by a general formula, Li₂MPO₄F, Li₂MP₂O₇, and Li₅MO₄ (M=Feor Mn), a perovskite fluoride such as NaFeF₃ and FeF₃, a metalchalcogenide (a sulfide, a selenide, and a telluride) such as TiS₂ andMoS₂, a lithium-containing material with an inverse spinel crystalstructure such as LiMVO₄, a vanadium oxide (e.g., V₂O₅, V₆O₁₃, andLiV₃O₈), a manganese oxide, and an organic sulfur compound can be usedas the positive electrode active material, for example.

In the case where the carrier ions are alkali metal ions other thanlithium ions or alkaline-earth metal ions, the following may be used asthe positive electrode active material: a compound or a material whichis obtained by substituting an alkali metal (e.g., sodium or potassium)or an alkaline-earth metal (e.g., calcium, strontium, barium, beryllium,or magnesium) for lithium in any of the above-described lithiumcompounds or lithium-containing materials.

In this embodiment, graphene is used as a conductive additive of theelectrode. Graphene oxide with a high dispersion property is used as araw material, and mixed and kneaded with the active material, a binder,a polar solvent (also referred to as a dispersion medium), and others.The mixture is called slurry.

First, the slurry containing the electrode active material particle 202is applied onto a base 200 and dried. Then, the graphene oxide isreduced and dried in a reduction atmosphere. As the base 200, apolyimide film tape is used. FIG. 3A is a cross-sectional schematic viewof the state in which the slurry containing the electrode activematerial particle 202 and the graphene oxide flakes is applied onto thebase 200 and dried, the graphene oxide is reduced, and then the grapheneoxide is dried in the reduction atmosphere. There is no particularlimitation on a method for reducing the graphene oxide. In thisembodiment, the graphene oxide is at least immersed in a reducingsolution containing ascorbic acid and water. Note that the pH of thereducing solution is higher than or equal to 4 and lower than or equalto 11.

As illustrated in FIG. 3A, a network for electron conduction by grapheneflake 205 is formed between the electrode active material particles 202.Consequently, an electrode for a storage battery in which grapheneelectrically connects the electrode active material particles to eachother can be formed. In FIG. 3A, the electrode active material particles202 are electrode active material particles made of secondary particleshaving an average particle diameter and a particle size distribution.For this reason, the electrode active material particles 202 areschematically illustrated as spheres in FIG. 3A; however, the shape ofthe electrode active material particle 202 is not limited to this shape.

Graphene oxide used as a raw material of graphene is a polar materialhaving a functional group such as an epoxy group, a carbonyl group, acarboxyl group, or a hydroxyl group; this makes it possible to form thenetwork. In graphene oxide in a polar solvent, oxygen in the functionalgroup is negatively charged; hence, graphene oxide flakes do not easilyaggregate but strongly interact with the polar solvent such asN-methyl-2-pyrrolidone (NMP). Thus, the functional group such as anepoxy group in the graphene oxide interacts with the polar solvent,which probably prevents aggregation among graphene oxide flakes,resulting in uniform dispersion of the graphene oxide flakes in thepolar solvent.

When graphene oxide is used as a raw material of a conductive additiveas described above, the graphene oxide has a high dispersion property ina polar solvent but has extremely low electric conductivity and thusdoes not function as the conductive additive without any change. Forthis reason, in forming an electrode for a storage battery, after atleast an active material and graphene oxide are mixed, the grapheneoxide needs to be reduced to form graphene with high electricconductivity.

Examples of a method for reducing graphene oxide are reduction treatmentwith heating (hereinafter referred to as thermal reduction treatment),electrochemical reduction treatment performed by application of apotential at which graphene oxide is reduced in an electrolytic solution(hereinafter referred to as electrochemical reduction), and reductiontreatment using a chemical reaction caused with a reducing agent(hereinafter referred to as chemical reduction).

The graphene 205 is formed by reducing the graphene oxide through theelectrochemical reduction or the chemical reduction and then, a metalfilm 201 is formed by sputtering as illustrated in FIG. 3B.

As illustrated in FIG. 3B, the metal film 201 bonds adjacent particleswith each other in the electrode active material particles 202. Further,as illustrated in FIG. 3B, the metal film 201 fills at least part of aspace between the electrode active material particles 202.

Then, as illustrated in FIG. 3C, the base 200 that is the polyimide filmtape is separated from the electrode active material particles 202.

Through the above steps, a positive electrode of a lithium ion secondarybattery, which is illustrated in FIG. 3D, can be formed.

The use of the metal film 201 as a current collector can reduce thethickness of the positive electrode. In addition, to increase theadhesion between the current collector and the electrode active materialparticle 202, the metal film 201 formed by sputtering is used as abuffer layer; thus, the lithium-ion secondary battery can have highreliability.

If necessary, a step of electrically connecting a current collector tothe metal film 201 may be performed in addition to the above steps.

This embodiment can be freely combined with Embodiment 1. For example,the negative electrode obtained in Embodiment 1, the positive electrodeobtained in this embodiment, a separator, and an electrolytic solutionare used to fabricate a thin lithium-ion secondary battery, in whichcase the adhesion between the current collectors and the electrodeactive materials is increased to enable high reliability.

Embodiment 3

In this embodiment, an example of a method for forming an electrode of alithium-ion secondary battery is described below. In the method, aplastic film whose surface is provided with a film containing siliconoxide is used as a base.

First, an ethyl silicate solution is applied onto a base 10 a and dried,whereby a film 10 b containing silicon oxide is formed on a surface ofthe base 10 a. As the base 10 a, polyethylene terephthalate (PET) isused.

Next, slurry containing an electrode active material 12 is applied anddried as illustrated in FIG. 4A. In addition to the electrode activematerial 12, a conductive additive, a NMP solvent, polyvinylidenefluoride, and the like are mixed in the slurry. The slurry is applied tohave a predetermined thickness with the use of a coating device such asa slot die coater.

Next, a metal film 11 is formed by sputtering as illustrated in FIG. 4B.For the metal film 11, titanium or copper is used.

Next, the film 10 b containing silicon oxide and the electrode activematerial 12 are separated at the interface therebetween.

Next, a current collector 14 whose surface is provided with a resin film13 of, for example, polyvinylidene fluoride or styrene-butadiene rubberis formed. As the current collector 14, metal foil such as aluminum foiland copper foil is used.

As illustrated in FIG. 4C, the current collector 14 and the metal film11 are attached to each other with the resin film 13 as an adhesive,whereby the current collector 14 is electrically connected to the metalfilm 11.

Through the above steps, the electrode of the lithium-ion secondarybattery can be formed.

To increase the adhesion between the current collector 14 and theelectrode active material 12, the metal film 11 formed by sputtering isused as a buffer layer; thus, the lithium-ion secondary battery can havehigh reliability.

The above process is an example and not particularly limited. Althoughthe current collector 14 and the metal film 11 are attached after thebase 10 a is separated in the above process, the base 10 a may beseparated after the metal film is formed and the current collector 14 isattached thereto, for example.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 4

In this embodiment, examples of the structure of a storage battery usingelectrodes for a storage battery formed by any of the formation methodsdescribed in Embodiments 1 to 3 are described with reference to FIGS. 5Ato 5C, FIG. 6, and FIGS. 7A and 7B.

(Coin-Type Storage Battery)

FIG. 5A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 5B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of, for example, polypropylene. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308. A separator 310 and anelectrolytic solution (not illustrated) are included between thepositive electrode active material layer 306 and the negative electrodeactive material layer 309.

As the negative electrode 307, an electrode for a storage battery formedby the method for forming an electrode for a storage battery, which isone embodiment of the present invention and is described in Embodiment1, can be used. As the positive electrode 304, an electrode for astorage battery formed by the method for forming an electrode for astorage battery, which is one embodiment of the present invention and isdescribed in Embodiment 2, can be used.

As the separator 310, an insulator such as cellulose (paper),polyethylene with pores, and polypropylene with pores can be used.

For an electrolyte salt of the electrolytic solution, a materialcontaining carrier ions is used. Typical examples of the supportingelectrolyte are lithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄,LiCF₃SO₃, Li(CF₃SO₂)₂N, and Li(C₂F₅SO₂)₂N. These supporting electrolytesmay each be used alone or two or more of them may be used in anappropriate combination and in an appropriate ratio.

Note that when carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, instead of lithium in the abovelithium salts, an alkali metal (e.g., sodium and potassium) or analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, andmagnesium) may be used for the electrolyte.

For a solvent of the electrolytic solution, a material in which carrierions can transfer is used. As the solvent of the electrolytic solution,an aprotic organic solvent is preferably used. Typical examples of theaprotic organic solvents include ethylene carbonate (EC), propylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, and tetrahydrofuran, and one or more ofthese materials can be used. When a gelled high-molecular material isused as the solvent of the electrolytic solution, safety against liquidleakage and the like is improved. Further, the storage battery can bethinner and more lightweight. Typical examples of the gelledhigh-molecular material include a silicone gel, an acrylic gel, anacrylonitrile gel, polyethylene oxide, polypropylene oxide, and afluorine-based polymer. Alternatively, the use of one or more of ionicliquids (room temperature molten salts) which have features ofnon-flammability and non-volatility as the solvent of the electrolyticsolution can prevent the storage battery from exploding or catching fireeven when the storage battery internally shorts out or the internaltemperature increases owing to, for example, overcharging.

Instead of the electrolytic solution, a solid electrolyte containing aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte containing ahigh-molecular material such as a polyethylene oxide (PEO)-basedhigh-molecular material may alternatively be used as the electrolyte. Inthe case of using the solid electrolyte, a separator or a spacer is notnecessary. Further, the battery can be entirely solidified; therefore,there is no possibility of liquid leakage and thus the safety of thebattery is dramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having corrosion resistance to an electrolytic solution, such asnickel, aluminum, and titanium, an alloy of such a metal, and an alloyof such a metal and another metal (e.g., stainless steel) can be used.Alternatively, the positive electrode can 301 and the negative electrodecan 302 are preferably coated with, for example, nickel or aluminum inorder to prevent corrosion caused by the electrolytic solution. Thepositive electrode can 301 and the negative electrode can 302 areelectrically connected to the positive electrode 304 and the negativeelectrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 5B, the positive electrode can 301, the positiveelectrode 304, the separator 310, the negative electrode 307, and thenegative electrode can 302 are stacked in this order with the positiveelectrode can 301 positioned at the bottom, and the positive electrodecan 301 and the negative electrode can 302 are subjected to pressurebonding with the gasket 303 interposed therebetween. In such a manner,the coin-type storage battery 300 is fabricated.

Here, a current flow in charging a battery is described with referenceto FIG. 5C. When a battery using lithium is regarded as a closedcircuit, lithium ions move and a current flows in the same direction.Note that in the battery using lithium, an anode and a cathode changeplaces in charge and discharge, and an oxidation reaction and areduction reaction occur on the corresponding sides; hence, an electrodewith a high redox potential is called a positive electrode and anelectrode with a low redox potential is called a negative electrode. Forthis reason, in this specification, the positive electrode is referredto as a “positive electrode” and the negative electrode is referred toas a “negative electrode” in all the cases where charge is performed,discharge is performed, a reverse pulse current is supplied, and acharging current is supplied. The use of the terms “anode” and “cathode”related to an oxidation reaction and a reduction reaction might causeconfusion because the anode and the cathode change places at the time ofcharging and discharging. Thus, the terms “anode” and “cathode” are notused in this specification. If the terms “anode” or “cathode” is used,whether it is at the time of charging or discharging is noted andwhether it corresponds to a positive electrode or a negative electrodeis also noted.

Two terminals in FIG. 5C are connected to a charger, and a storagebattery 400 is charged. As the charge of the storage battery 400proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 5C is the direction in which a current flowsfrom the one terminal outside the storage battery 400 to a positiveelectrode 402, flows from the positive electrode 402 to a negativeelectrode 404 in the storage battery 400, and flows from the negativeelectrode 404 to the other terminal outside the storage battery 400. Inother words, a current flows in the direction of a flow of a chargingcurrent.

(Laminated Storage Battery)

Next, an example of a laminated storage battery is described withreference to FIG. 6.

A laminated battery cell 500 illustrated in FIG. 6 includes a positiveelectrode 503 including a positive electrode current collector 501 and apositive electrode active material layer 502, a negative electrode 506including a negative electrode current collector 504 and a negativeelectrode active material layer 505, a separator 507, an electrolyticsolution 508, and an exterior body 509. The separator 507 is providedbetween the positive electrode 503 and the negative electrode 506 in theexterior body 509. The exterior body 509 is filled with the electrolyticsolution 508. Furthermore, a flexible secondary battery including aflexible electrode can be provided.

In the laminated storage battery 500 illustrated in FIG. 6, the positiveelectrode current collector 501 and the negative electrode currentcollector 504 also function as terminals for electrical contact with anexternal portion. For this reason, each of the positive electrodecurrent collector 501 and the negative electrode current collector 504is arranged so that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposedoutside the exterior body 509.

As the exterior body 509 in the laminated storage battery 500, alaminate film having a three-layer structure can be used, for example.In the three-layer structure, a highly flexible metal thin film of forexample, aluminum, stainless steel, copper, and nickel is provided overa film formed of a material such as polyethylene, polypropylene,polycarbonate, ionomer, and polyamide, and an insulating synthetic resinfilm of, for example, a polyamide-based resin and a polyester-basedresin is provided as the outer surface of the exterior body over themetal thin film. With such a three-layer structure, permeation of anelectrolytic solution and a gas can be blocked and an insulatingproperty and resistance to the electrolytic solution can be provided.

(Cylindrical Storage Battery)

Next, an example of a cylindrical storage battery is described withreference to FIGS. 7A and 7B. As illustrated in FIG. 7A, a cylindricalsecondary battery 600 includes a positive electrode cap (battery cap)601 on its top surface and a battery can (outer can) 602 on its sidesurface and bottom surface. The positive electrode cap 601 and thebattery can 602 are insulated from each other with a gasket (insulatingpacking) 610.

FIG. 7B is a schematic view of a cross-section of the cylindricalstorage battery. Inside the battery can 602 having a hollow cylindricalshape, a battery element in which a strip-like positive electrode 604and a strip-like negative electrode 606 are wound with a separator 605positioned therebetween is provided. Although not illustrated, thebattery element is wound around a center pin. One end of the battery can602 is close and the other end thereof is open. For the battery can 602,a metal having corrosion resistance to an electrolytic solution, such asnickel, aluminum, or titanium, an alloy of such a metal, or an alloy ofsuch a metal and another metal (e.g., stainless steel or the like) canbe used. Alternatively, the battery can 602 is preferably covered withnickel, aluminum, or the like in order to prevent corrosion caused bythe electrolytic solution. Inside the battery can 602, the batteryelement in which the positive electrode, the negative electrode, and theseparator are wound is positioned between a pair of insulating plates608 and 609 which face each other. Further, a nonaqueous electrolyticsolution (not illustrated) is injected inside the battery can 602provided with the battery element. As the nonaqueous electrolyticsolution, a nonaqueous electrolytic solution which is similar to thoseof the above coin-type storage battery and the laminated power storagedevice can be used.

The positive electrode 604 and the negative electrode 606 can be formedin a manner similar to that of the positive electrode and the negativeelectrode of the coin-type storage battery described above; however, thedifference lies in that, active material layers are formed on both sidesof a current collector in each electrode because the positive electrodeand the negative electrode of the cylindrical storage battery are wound.A positive electrode terminal (positive electrode current collectinglead) 603 is connected to the positive electrode 604, and a negativeelectrode terminal (negative electrode current collecting lead) 607 isconnected to the negative electrode 606. Both the positive electrodeterminal 603 and the negative electrode terminal 607 can be formed usinga metal material such as aluminum. The positive electrode terminal 603and the negative electrode terminal 607 are resistance-welded to asafety valve mechanism 612 and the bottom of the battery can 602,respectively. The safety valve mechanism 612 is electrically connectedto the positive electrode cap 601 through a positive temperaturecoefficient (PTC) element 611. The safety valve mechanism 612 cuts offelectrical connection between the positive electrode cap 601 and thepositive electrode 604 when the internal pressure of the battery exceedsa predetermined threshold value. The PTC element 611 is a heat sensitiveresistor whose resistance increases as temperature rises, and controlsthe amount of current by increase in resistance to prevent abnormal heatgeneration. Note that barium titanate (BaTiO₃)-based semiconductorceramic or the like can be used for the PTC element.

Note that in this embodiment, the coin-type storage battery, thelaminated storage battery, and the cylindrical storage battery are givenas examples of the storage battery; however, any of storage batterieswith a variety of shapes, such as a sealed storage battery and arectangular storage battery, can be used. Further, a structure in whicha plurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or rolled may be employed.

As the positive electrodes and the negative electrodes of the storagebattery 300, the storage battery 500, and the storage battery 600, whichare described in this embodiment, electrodes formed by the method forforming an electrode for a storage battery which is one embodiment ofthe present invention are used. Thus, the discharge capacity of thestorage batteries 300, 500, and 600 can be increased.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 5

In this embodiment, examples of electronic devices including any of thestorage batteries illustrated in the above embodiments are describedwith reference to FIGS. 8A and 8B and FIGS. 9A and 9B.

Examples of electronic devices including storage batteries are camerassuch as digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cellular phones or portable telephonedevices), portable game consoles, portable information terminals, andaudio reproducing devices. FIGS. 8A and 8B illustrate specific examplesof these electronic devices.

FIG. 8A illustrates an example of a mobile phone. A mobile phone 800 isprovided with a display portion 802 incorporated in a housing 801, anoperation button 803, a speaker 805, a microphone 806, and the like. Theuse of a storage battery 804 of one embodiment of the present inventionin the mobile phone 800 results in weight reduction.

When the display portion 802 of the mobile phone 800 illustrated in FIG.8A is touched with a finger or the like, data can be input into themobile phone 800. Users can make a call or text messaging by touchingthe display portion 802 with their fingers or the like.

There are mainly three screen modes for the display portion 802. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion802 so that text displayed on a screen can be inputted.

When a sensing device including a sensor such as a gyroscope and anacceleration sensor for detecting inclination is provided in the mobilephone 800, display on the screen of the display portion 802 can beautomatically changed in direction by determining the orientation of themobile phone 800 (whether the mobile phone 800 is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 802 oroperating the operation button 803 of the housing 801. Alternatively,the screen modes may be switched depending on the kind of the imagedisplayed on the display portion 802. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 802 is detected and the input by touch on thedisplay portion 802 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 802 can function as an image sensor. For example, animage of a palm print, a fingerprint, or the like is taken with thedisplay portion 802 touched with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, and thelike can be taken.

FIG. 8B illustrates a smart watch. The smart watch can include a housing702, a display panel 704, operation buttons 711 and 712, a connectionterminal 713, a band 721, a clasp 722, and so on. The use of the storagebattery of one embodiment of the present invention in the smart watchresults in weight reduction.

The display panel 704 mounted in the housing 702 serving as a bezelincludes a non-rectangular display region. The display panel 704 candisplay an icon 705 indicating time and another icon 706.

The smart watch in FIG. 8B can have a variety of functions, for example,a function of displaying a variety of information (e.g., a still image,a moving image, and a text image) on a display portion, a touch panelfunction, a function of displaying a calendar, date, time, and the like,a function of controlling processing with a variety of software(programs), a wireless communication function, a function of beingconnected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading program or data stored in a recording medium and displayingthe program or data on a display portion.

The housing 702 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and so on.

A vacuum cleaner 901 illustrated in FIG. 9A includes a thin secondarybattery 902 which is curved. The thin secondary battery is illustratedin FIG. 9B. In the vacuum cleaner 901, a space for collecting dust ispreferably as large as possible. For this reason, it is effective to usethe thin secondary battery 902 which is curved to fit the shape of anouter surface of the vacuum cleaner.

The thin secondary battery 902 can be fabricated by a method forfabricating the laminated storage battery described in Embodiment 4 withthe use of any of the electrodes for a storage battery described inEmbodiments 1 to 3.

The thin secondary battery 902 is a laminated secondary battery andfixed to be curved. The vacuum cleaner 901 includes a display portion903 that displays, for example, the remaining amount of power in thethin secondary battery 902. A display area of the display portion 903 iscurved to fit the shape of the outer surface of the vacuum cleaner. Thevacuum cleaner includes a power cord. When the thin secondary battery902 is charged to have sufficient power, the power cord can be removedfrom the receptacle to use the vacuum cleaner. The thin secondarybattery 902 may be charged wirelessly without using the power cord.

Note that the structure and the like described in this embodiment can beused as appropriate in combination with any of the structures and thelike in the other embodiments.

EXPLANATION OF REFERENCE

10 a: base, 10 b: film, 11: metal film, 12: electrode active material,13: resin film, 14: current collector, 100: base, 101: metal film, 102:electrode active material, 104: current collector, 200: base, 201: metalfilm, 202: electrode active material particle, 205: graphene, 300:storage battery, 301: positive electrode can, 302: negative electrodecan, 303: gasket, 304: positive electrode, 305: positive electrodecurrent collector, 306: positive electrode active material layer, 307:negative electrode, 308: negative electrode current collector, 309:negative electrode active material layer, 310: separator, 400: storagebattery, 402: positive electrode, 404: negative electrode, 500: storagebattery, 501: positive electrode current collector, 502: positiveelectrode active material layer, 503: positive electrode, 504: negativeelectrode current collector, 505: negative electrode active materiallayer, 506: negative electrode, 507: separator, 508: electrolyticsolution, 509: exterior body, 600: storage battery, 601: positiveelectrode cap, 602: battery can, 603: positive electrode terminal, 604:positive electrode, 605: separator, 606: negative electrode, 607:negative electrode terminal, 608: insulating plate, 609: insulatingplate, 611: PTC element, 612: safety valve mechanism, 702: housing, 704:display panel, 705: icon, 706: icon, 711: operation button, 712:operation button, 713: connection terminal, 721: band, 722: clasp, 800:mobile phone, 801: housing, 802: display portion, 803: operation button,804: storage battery, 805: speaker, 806: microphone, 901: vacuumcleaner, 902: secondary battery, 903: display portion, 1700: curvedsurface, 1701: plane surface, 1702: curve, 1703: radius of curvature,1704: center of curvature, 1800: center of curvature, 1801: film, 1802:radius of curvature, 1803: film, 1804: radius of curvature, and 1805:component including electrodes and an electrolytic solution.

This application is based on Japanese Patent Application serial no.2013-088165 filed with Japan Patent Office on Apr. 19, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for forming an electrode, comprising thesteps of: forming an active material layer over a base; forming a metalfilm over and in contact with the active material layer; and separatingthe base from the active material layer after forming the metal film. 2.The method for forming an electrode according to claim 1, furthercomprising the step of: attaching a current collector to the metal filmafter separating the base after separating the base.
 3. The method forforming an electrode according to claim 2, wherein the current collectoris electrically connected to the metal film and the active materiallayer by the attaching step.
 4. The method for forming an electrodeaccording to claim 2, wherein the current collector is attached to themetal film with an adhesive film.
 5. The method for forming an electrodeaccording to claim 4, wherein the adhesive film is a resin film.
 6. Themethod for forming an electrode according to claim 1, wherein the metalfilm is formed by a sputtering method.
 7. The method for forming anelectrode according to claim 1, wherein the metal film comprises one oftitanium and copper
 8. The method for forming an electrode according toclaim 1, wherein the active material layer comprises active materialparticles.
 9. The method for forming an electrode according to claim 8,wherein the active material particles each comprise carbon or silicon.10. The method for forming an electrode according to claim 1, whereinthe base comprises a plastic film.
 11. The method for forming anelectrode according to claim 1, wherein the base comprises a siliconoxide film on a surface of the base.
 12. A method for fabricating asecondary battery, comprising the steps of: forming an active materiallayer over a base, the active material layer comprising a plurality ofactive material particles; forming a metal film over and in contact withthe active material layer; separating the base from the active materiallayer to form a first electrode after forming the metal film; andproviding the first electrode, a second electrode and an electrolyte inan exterior body.
 13. The method for fabricating a secondary batteryaccording to claim 12, wherein the metal film is formed by a sputteringmethod.
 14. The method for fabricating a secondary battery according toclaim 12, wherein the active material layer comprises a first particleand a second particle which are adjacent to each other, wherein themetal film extends into a space between the first particle and thesecond particle.
 15. The method for fabricating a secondary batteryaccording to claim 12, wherein the metal film fills a depression on asurface of the active material layer.
 16. The method for fabricating asecondary battery according to claim 12, wherein the base comprises aplastic film.
 17. The method for fabricating a secondary batteryaccording to claim 12, wherein the base comprises a silicon oxide filmon a surface of the base.
 18. The method for fabricating a secondarybattery according to claim 12, wherein the electrolyte comprises anelectrolytic solution and a gelled polymer material.
 19. The method forfabricating a secondary battery according to claim 12, wherein thesecondary battery is flexible.